rates of reactions Flashcards
What is the rate equation
Rate = k [A]^x[B]^y , where A and B are reactants
What are the units for rate
Usually moldm-3 however can be units on y axis / units on x axis -1
What is an order of reaction
In a reaction , different reactants have different orders and each will effect the rate of reaction differently . Zero order will not have an effect on rate , 1 st order will have a proportional effect on rate , and 2nd order will have a squared effect on rate
How do you state what order a reactant is
- a reaction is ………. Order with respect to [X]
How do you work out the overall order of a reaction
Add the powers
What are the different ways rate can be measured
- change in PH of a reaction , eg if H+ ions are used up or produced
-amount of mass lost (gas being produced)
-volume of gas produced
-colorimeter - measures the absorbance of light , the more concentrated , the more pigment and the more light absorbed
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What is the rate constant
The rate constant K , allows us to equate rate and concentration
What happens with K
k is only fixed at a particular temperature , if temperature changes so does the rate constant . K increases when temperature increases . The larger the value of ‘k’ the faster the rate of reaction
Because particles have more kinetic energy and frequency of collisions increases
- concentrations of substances remain constant so k must change .
How do you find the units and value for k
- through experimental data find the rate equation
- rearrange for k and sub in units
- use experimental data to sub in values to find the rate constant
how do you calculate Ea from a graph
- first we find the 1/T values and the lnk values and plot 1/T(x) against lnk to find the gradient . the gradient we then x 8.314 to give the activation energy in joules
then we /1000 to get the activation energy in kj
what is the defintion for an intermediate
a species formed in one step of a multi-step reaction that is used up in a subsequent step , and is not seen as either a reactant or product of the overall reaction
what is the rate determining step
the slowest step in a reaction mechanism
what does the overall rate equation equal
the rate of the slowest step
what do you include in the rate equation
- the rate determining step reactants , and any reactants before this
what does the order with respect to a particular reactant also tell us
the number of molecules of that reactant that participate in the rate determining step
what is the Arrhenius equation
an expression that describes how the rate constant (k) for a given reaction varies with temperature and activation energy
k=Ae ^ -Ea/RT
k is the rate constant
A is the pre - exponential factor
EA is the activation energy in joules
R is the ideal gas constant
T is the temperature
what can we do with this equation
we can log both sides (ln) which allows us to plot a graph and find the activation energy
what does A mean
A takes into account the frequency of collisions with the correct orientation
